1. Field of the Invention
The present invention relates to a sensing device and, more particularly, to a sensing device for accurately sensing the presence/absence of a target object by a sensor. The present invention further relates to an original sensing device and, more particularly, to an original sensing device in a copying machine with an automatic document feeder, which accurately senses the presence/absence of originals placed on an original tray when each of the originals stacked on the original tray is moved to a copying machine main body and image information on the original is read.
2. Related Background Art
FIG. 1 is a schematic view showing the main parts of a copying machine as an image forming apparatus having an automatic document feeder with a conventional original sensing device. General functions and operations of the automatic document feeder will be described below with reference to FIG. 1.
First, a sensor (original sensing device) 103 senses the presence/absence of originals 204 stacked on an original tray 101 of the document. feeder. An output signal from this sensor 103 is input to a copying machine 104. When an operator operates a copy start switch (not shown) of the copying machine 104, paper feed rollers 105, 106, and 107 and a paper feed belt 108 rotate in the direction of an arrow in FIG. 1. A stop blade 109 for preventing multiple sheet feeding separates the lowermost one of the originals 204 and feeds this original 204 to the upper left portion in FIG. 1. The fed original 204 is conveyed onto an original glass table 122 by the rotation of an original conveyor roller 111, as a conveyor means which has started rotating in the direction shown by an arrow by the copy start operation, and by the rotation of a press roller 110 and an original conveyor belt 120 which rotate in accordance with the rotation of the original conveyor roller 111. When the original 204 is set in a predetermined position on the original glass table 122, the rotation of the original conveyor belt 120 is stopped. Rollers 121 behind the original conveyor belt 120 press the original 204 with appropriate pressure against the original glass table 122, and the copying machine 104 (details of its optical system are not shown) starts reading an image. When the exposure is complete, the copying machine 104 generates a signal to rotate the original conveyor belt 120 and rollers 141, 142, and 143 and deliver the original 204 subjected to the image reading onto the original tray 101.
FIG. 2 is a schematic view showing the major parts of the conventional original sensing device. FIG. 3 is a schematic view showing the main components of a projector shown in FIG. 3.
Referring to FIG. 2, a projector 211 has, e.g., a light-emitting diode (LED) as a light source. A light-shielding window member 201 has openings 205 and 206. A transparent dust cover 202 is placed on this light-shielding window member 201. Portions 203 on this dust cover 202 are part of an original tray case. The originals 204 are placed on the original tray 101. A photodetector 212 has a sensor (photosensor). A printed board 213 fixes the projector 211 and the photodetector 212.
Referring to FIG. 3, a light-emitting chip 11 of the LED is usually encapsulated with a transparent resin. A ring-like reflector 12 is placed near this light-emitting chip 11. When electrodes 15 supply electric power, the whole light-emitting chip 11 emits light, and the reflector 12 reflects a portion of the emitted light beam toward a portion above the light-emitting chip 11. Since a dome-like lens 14 is placed above the light-emitting chip 11, the light beam entering this lens 14 slightly decreases the diffusion angle when emerging from the lens 14 and further points upward as a light beam 16 shown in FIG. 3. On the other hand, light beams entering a cylindrical portion 13, rather than the lens 14, are largely refracted because the angle of incidence to the cylindrical portion 13 is large. Consequently, these light beams obliquely point upward as light beams 17 shown in FIG. 3.
Referring to FIG. 2, of the light beams emitted upward from the projector 211, those passing through the opening 205 in the light-shielding window member 201 illuminate the surface of the original 204 through the transparent dust filter 202. A curve A in FIG. 4 indicates the light amount distribution in the illuminated portion on the surface of the original 204. The illuminating light amount is largest immediately above the light source. Of the light beams reflected by the surface of the original 204, those transmitted through the dust filter 202 and passing through the opening 206 in the light-shielding window member 201 irradiate the photoelectric surface through a resin lens 21 in the upper portion of the photodetector 212. A curve B in FIG. 4 indicates the sensitivity distribution on the surface of the original 204 obtained by the photodetector 212. The sensitivity is highest immediately above the photodetector 212.
When the original 204 is placed on the original tray 101 in the above arrangement, light beams from the projector 211 illuminate the surface of the original 204 as indicated by the curves A and B in FIG. 4. Of reflected light beams from the illuminated portion of the original 204, those passing through the opening 206 in the light-shielding window member 201 irradiate the photodetector 212. These light beams are photoelectrically converted by the photodetector 212 and converted into an electrical signal by an electronic circuit (not shown). When the original 204 does not exist on the original tray 101, no reflected light beams are produced, so neither light beams irradiate the photodetector 212, nor electrical signal is generated. With this arrangement, the presence/absence of the original 204 on the original tray 101 can be converted into an electrical signal.
In this conventional device, as shown in FIG. 4, a portion where the amount of illuminating light from the light source (projector) is largest is different from a portion where the sensitivity of light detection by the photodetector is highest. Therefore, the amount of light entering the photodetector is small even when an original exists on the original tray. To overcome this drawback, one of following means (1), (2), and (3) is conventionally used.
(1) Increase the light emission amount of the light source of the projector to increase the amount of illuminating light to an original.
(2) Raise the sensitivity of the photodetector.
(3) Make the illuminating light amount largest and the sensitivity highest in close positions or in the same position.
Unfortunately, these means have the following problems.
Means (1) requires an expensive light source because a high-output light source is necessary.
Means (2) is readily influenced by external light such as a ceiling illuminating lamp because the sensitivity of the photodetector is raised. Accordingly, light beams sometimes enter the sensor although no original exists on the original tray, and a detection error occurs in some cases.
Means (3) will be described in detail below.
(A) The light source and the sensor can also be inclined with respect to the original tray surface (original surface). If this is the case, however, the light source and the sensor can no longer be fixed on the same plane of one printed board. The use of a plurality of printed boards requires a connector and results in an expensive device. Also, assembly of these parts into the device requires much labor and increases the assembly cost.
(B) FIGS. 5 and 6 show a conventional device when the countermeasure according to means (3) above is practiced. Referring to FIGS. 5 and 6, a condenser lens 221 has a convex section. In FIG. 5, light beams from the light-emitting chip of the LED of the projector 211 are fed into the left-hand side, in FIG. 5, of the condenser lens 211 and bent to the right toward a portion above the photodetector 212, thereby illuminating the original 204. The reflected light beams from the original 204 are passed through the right-hand side, in FIG. 5, of the condenser lens 221 so that these light beams enter the photodetector 212, thereby causing the light beams to point toward the sensor. This makes the position where the light beams from the projector 211 irradiate the original 204 the came as the position on the surface of the original 204 where the sensitivity of the photodetector 212 is highest. Consequently, the sensing efficiency increases.
Means (3) described above, however, has the following problems.
(C) An upper surface 221a of the condenser lens 221 reflects some light beams from the projector 211. Especially when light beams transmitted through the cylindrical portion 13 of the projector 211 enter the upper portion of the condenser lens 221, a slightly generated reflected light beam points in the direction of the photodetector 212 as shown in FIG. 6 and enters the photodetector 212 regardless of the presence/absence of the original 204. To eliminate this reflected light beam, a more expensive countermeasure is necessary, e.g., an antireflection coating must be formed on the upper surface 221a of the condenser lens 221.
(D) As shown in FIG. 7, slight paper dust particles falling from the original 204 are deposited on the dust filter 202 with the lapse of time. This intercepts the detecting optical path to lower the sensitivity or causes a detection error if reflected light beams from paper dust particles are detected as the existence of the original 204. Even after a serviceman cleans the dust filter 202, paper dust articles sometimes remain in corners and move from the corners to the middle of the optical path due to vibrations when the lower copying apparatus operates and thereby cause a detection error.